ShellAppCache: add cache to help keep I/O off main thread

A number of things can result in doing I/O on the main thread such as
loading GAppInfo or folder translations. The ShellAppCache provides a
layer of abstraction around those things so that we can keep that work
off the main thread by delaying a short while and processing the work
synchronously off-main-thread.

The results can then be marshalled back to the main thread for use by
the rest of the system via the ShellAppCache::changed() signal.

src/shell-app-cache.h

@@ -0,0 +1,19 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+#ifndef __SHELL_APP_CACHE__
+#define __SHELL_APP_CACHE__
+
+#include <gio/gio.h>
+#include <gio/gdesktopappinfo.h>
+
+#define SHELL_TYPE_APP_CACHE (shell_app_cache_get_type())
+
+G_DECLARE_FINAL_TYPE (ShellAppCache, shell_app_cache, SHELL, APP_CACHE, GObject)
+
+ShellAppCache   *shell_app_cache_get_default      (void);
+GList           *shell_app_cache_get_all          (ShellAppCache *cache);
+GDesktopAppInfo *shell_app_cache_get_info         (ShellAppCache *cache,
+                                                   const char    *id);
+char            *shell_app_cache_translate_folder (ShellAppCache *cache,
+                                                   const char    *name);
+
+#endif /* __SHELL_APP_CACHE__ */